Tissue factor, an integral membrane protein, is the primary triggering agent of the clotting cascade. Its ligand, factor VII/VIIa, is a serine proteinase that can exist as an inert zymogen (factor VII) or active enzyme (factor VIIa). Both forms of factor VII circulate in plasma, averaging about 99% factor VII and 1% factor VIIa, with a wide range of individual variation (the latter from 0.1 % to 1.8%). A critical regulatory step in controlling the clotting cascade in both normal hemostasis and thrombotic disease is the assembly of the tissue factor/factor VIIa complex on cell surfaces. Because of this, and because elevated plasma factor VII activity is implicated as a risk factor for ischemic heart disease, a long-term goal of this laboratory is to understand how the tissue factor/factor VIIa system is regulated and how this process is altered in thrombotic disease. This project will investigate the role of plasma factor VIIa in hypercoagulable states, and will examine how changes in phospholipid content and antiphospholipid antibodies alter the function of the tissue factor/factor VIIa complex. Specifically the project will address the following questions: (1) How does phospholipid composition regulate the properties of tissue factor/factor VIIa? (2) How do anti-phospholipid antibodies alter tissue factor/factor VIIa function? (3) How do protein cofactors and phospholipid modulate proteolytic activation of factor VII ? (4) What is the relationship between plasma factor VIIa levels and the presence of Arg353 versus Gln353 alleles of factor VII? What are the activation properties of Gln353 factor VII? and (5) How do plasma factor VIIa levels vary in normal individuals? In addition, a collaborative study with Project 1 (Project leader: G. Raskob) will directly test hypotheses that plasma factor VIIa levels (especially rebound in plasma factor VIIa following cessation of warfarin therapy) predict risk of re- thrombosis in patients with previously documented deep vein thrombosis. Achieving the goals of this project will provide basic knowledge about how the activity of tissue factor and factor VIIa can be modulated. It will also provide knowledge about the participation of factor VIIa in hypercoagulable states.